1. Field of the Invention
This invention relates to electromagnetic wave cavity filters having a dielectric resonator, and to combinations of such filters within a common shield.
2. Prior Art
An electromagnetic resonator is a device which allows one electromagnetic frequency to pass through it while rejecting all other frequencies. Such resonators are common elements in communications systems. In the UHF and microwave region, or the frequencies above 300 megahertz, the resonators required to give adequate frequency selectivity and power transmission take the form of hollow metallic cylinders. These resonators can occupy a large volume if high selectivity and low losses are required. In general, a higher degree of selectivity requires a larger resonator, possessing a higher Q factor. The Q factor, defined as the ratio of the energy stored in the resonator to that dissipated per frequency cycle, is the common measure of a resonator's performance. In complex systems, the volume occupied by a resonator of desired Q factor is excessive. For example, a transmitter system operating at 900 MHz which combines 16 transmitters into one antenna requiring Q values of 15,000 requires 300 cubic inches of space per channel for the resonators.
The use of a dielectric material having a high dielectric constant such as barium titanate enables reduction of the volume of the resonator by a factor of fifty with the same Q factor.
A dielectric resonator must be enclosed in an enclosure to reduce coupling to other resonators and to the outside environment. This aspect of resonator design is described in U.S. Pat. No. 4,241,322 issued Dec. 23, 1980 to Johnson et al. which is also generally descriptive of the prior art relating to this invention. All reference numerals recited in the remainder of this Prior Art section relate to Johnson et al.
Dielectric resonators are usually tunable within a band of frequencies. The exact frequency at which resonance occurs can be operator selected by rotation of a screw which raises and lowers the position of a flat plate held above the dielectric. Refer to Johnson et al. A dielectric resonator 11 is held by epoxy to a substrate 12, composed of a material which has low heat conductivity. (Col. 3, L45-50) A tuning plate 41 moves toward or away from dielectric resonator 11 to tune the response of the resonator. (Col. 4, L40-42).
During operation of the resonator, electromagnetic energy at input terminal 30 or 55 which oscillates at the resonance frequency will appear at output terminal 30 or 35. Electromagnetic energy which oscillates at other frequencies will be discriminated against by reflection within the resonant structure. Dielectric resonator 11 is cooled by conduction and convection in the air within the shield formed by housing 21,22. During temperature transients, heatup and cooldown, the Q factor may vary as may the resonance frequency.
Multiple filters, tuned to seperate resonance frequencies, may be grouped togather in a common assembly to facilitate connection to a common antenna. Johnson et al. illustrates a typical grouping.
It is an object of this invention to improve the heat transfer of a dielectric resonating filter.
It is a further object of this invention to improve the temperature stability of the resonance frequency of a dielectric filter.
It is a further object to facilitate grouping of multiple filters.